Hydrogen-based energy is the cleanest of the currently known energy sources, and it will undoubtedly play a part in the energy supply of this century. Heavy environmental pollution due to combustion of fossil fuel and depletion of non-renewable energy sources emerge as two serious problems. Hydrogen-based energy sources are considered to be the most promising candidates for solving these problems, as this kind of energy can replace fossil fuel in most applications. The biggest challenge in on-board hydrogen utilisation (i.e. as fuel for vehicle, portable computer, phone, etc.) is the low hydrogen storage capacity that existing systems possess. Development of hydrogen storage media is of great importance and research on this area is quite active throughout the world.
Currently, there are four systems for hydrogen storage [1, 2]: Liquid hydrogen, Compressed hydrogen gas, Cryo-adsorption system, and Metal hydride systems.
Applications of hydrogen in pure form (liquid hydrogen or compressed hydrogen gas) are mostly utilised for large-scale or stationary purposes, since the weight of containers for hydrogen is normally too prohibitive for uses where hydrogen is used in limited scope. For vehicular or any other portable applications, hydrogen stored in solid-state materials seems to be the best solution. Thus, cryo-adsorption systems and metal hydride systems are the two most promising systems.
The cryo-adsorption systems show advantages in moderate weight and volume. In this system, hydrogen molecules are physically bound to the surface of activated carbon at liquid nitrogen temperature. Under optimised conditions, the hydrogen storage capacity of activated carbon may reach 7 wt % based on the weight of activated carbon. The disadvantages of this system relate to the critical conditions required (i.e. cryogenic conditions).
Metal hydrides have been proposed as systems for hydrogen storage. Hydrogen is chemisorbed by metal or metal alloys with corresponding formation of metal hydrides. Two categories of metal alloys have been extensively explored: I) AB5 type, and ii) A2B type. LaNi5 is a good example of the first category. One molecule of LaNi5 can absorb about 6 hydrogen atoms at ambient temperature and high pressures to form LaNi5H6. Subsequent discharge of hydrogen can be achieved by reducing the hydrogen pressure. In this system, the hydrogen storage capacity is less than 1.5 wt %. The advantages of this type of metal alloy lie in the quick kinetics of hydrogen charge/discharge and the very good density of the materials, but the hydrogen storage capacity is unacceptable. Mg2Ni illustrates the A2B type of metal alloy. This kind of metal alloy can store more than 4 wt % of hydrogen, but suffers from higher operating temperature (above 300° C. for desorption, with an equilibrium hydrogen pressure of up to 1.0 bar), slow hydrogen charge and discharge kinetics and relatively low density. More recently, much effort has been made on material engineering of these metal alloys [3, 4], but no significant improvement has been made. Furthermore, the high cost of the metal alloys is another drawback.